Add support for tensor smoothing (`te`)

[matthewwardrop]

Patsy implementation: https://tedboy.github.io/patsy_doc/generated/patsy.te.html

I personally do not use this functionality, and so may eventually get around to this out of interest, but unless I hear of external interest, this is not going to be a high priority.

[david-waterworth]

This is definitely something I'm interested it, I'm currently using patsy to construct a design matric term by term and te is something I need (although I can probably bin one of the variables and treat it as categorical).

Ideally I'd also like to see support by mcgv's by for interaction between a spline and categorical variable - i.e. s(x, by=c) where x is continuous and c is categorical

Currently I do this in patsy by created a design matrix for each category then zero out the rows that aren't relevent.

[matthewwardrop]

Thanks for letting me know! This is useful feedback :).